And, Another Thing...
David le Comte wrote:
"...On two layer boards (with 5V CMOS logic in particular) it is very difficult to pass CISP-22 EMC tests without a well grounded flood plane.
In the 1980s it became a requirement for more and more categories of electronic equipment to be tested for EMC compliance.
From first hand experience, I have seen how just adding a flood plane to two-layer boards can reduce EMC by 20dB. (We had to revise existing boards to pass EMC tests during the late 80s and 90s)..." (See his entire comment on the Circuits Assembly blog link above)
That brings up a very good point. EMI. In the prototype/experimental world and the hobby world, there are so many cases where EMI isn't too much of an issue. We don't always think about it. If your project is going into a consumer or business consumer product, no questions, though, EMI is a big consideration. As David indicates, a grounded pour or plane can go a long ways toward keeping stray interference down.
I wonder if an Arduino could pass any EMI standards. Has anyone run it through a lab? Maybe with some good shielding. I bet the "Knight Rider" teeth I made for my pumpkin this Halloween (with a point to point wired PIC16F819) just radiates all over the place.
Pore, pour, pitiful me